•138 On the Barometer. 



temperature, is exhibited ; whereas it is in some degree 

 obscured, or suspended, during cloudy weather. 



The mercury in the baromeler is usually JLxed, at what- 

 ever height it may happen to be, with a concave surface, 

 ilvmng settled rain; and jixed with a convex surface, du- 

 ring settled fair weather. It is essential, in order to make 

 accurate observations on the barometer, that it be viewed 

 •with a magnifier, which is best when^/xW to the moveable 

 index. 



Fogs in the morning are more visible over land than over 

 water, because the greater coldness ol the land condenses 

 the vap07ir most ; but in the evening, over the water, be- 

 cause tvater is then the coldest. 



The mean height of the clouds above the earth's surface 

 is about three quarters of a mile; — the greatest height, 

 somewhat more than a mile — and the least height, about 

 three furlongs, and sometimes in thunder storms, even less. 



In order 1.0 make the most accurate observations on the 

 state of the weather, and to predict the various changes 

 which are about to take place in it, the observer should be 

 provided with a barometer, thermometer, and hygrometer, 

 each of the best construction. De Luc's hygrometer, I 

 think, is the best; — and in order, at all times, t*o be able to 

 notice the changes in the direction of the wind, which are 

 taking place, and which is extremely esseniial, it would be 

 expedient that a wind-vane be so constructed, as to exhibit 

 the direction of the wind within the room appropriated for 

 meteorological observations ; which, in defect of any better 

 means, might be contrived, by the axis of the tvi?id-vane, 

 without being continued so as to reach within the room, 

 having an index affixed to it. 



The I'ising of the barometer is a more certain indication 

 oi fair wecii her, than its ^/?2^iff^ is of rainy iveal her ; be- 

 cause it sinks for wind, as well as rain. Jf therefore, 

 whilst the barometer is sinking, the atmosphere still remain 

 clear, wind may be expected. 



In winter, spring, and autumn, a sudden falling of the 

 mercury presages high winds and storms ; but in summer^ 

 heavy shoivers, and sometimes thunder. 



The principal changes in the atmosphere take place 

 about the time of the vernal erjuinox, viz. from the iGth to 

 the 28th of March. 



Rain or snoiu may come during a ?iorlh and easterly 

 wind, whilst the barometer stands higher, than when the 

 wind is in the south or nest points j because in the former 



instance 



